<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script type="text/javascript">
    var a=parseInt(prompt("请输入一个数字"));
    var b;
    if (a>60){
        b=1;
        alert(b);
    }else if (a>70){
        b=2;
        alert(b);
    }else if (a>80){
        b=3;
        alert(b);
    }else if (a>90){
        b=4;
        alert(b);
    }
</script>
</body>
</html>